Optimize AppOpsManager.getFormattedStackTrace

Avoid redundant toString() calls for a given StackTraceElement when
fetching the formatted stack trace for an Op. This call is not trivially
cheap, and shows up in system stack samples, so make sure it only
happens once.

Bug: 308786093
Test: m + presubmit
Change-Id: I463addc4995b1553ff4b22e712208557ab4a663c